This invention relates generally to gate valves and deals more particularly with an expanding gate valve having floating valve seats which are restricted in their floating movement.
In a typical expanding gate valve, the gate assembly includes a cooperating gate and segment which are expanded by wedging action against the valve seats in both the open and closed positions of the valve. To prevent excessive dragging of the seats against the gate and segment, the gate assembly is collapsed or contracted inwardly of the seats during movement between the open and closed positions. It is usually preferred that the seats be mounted rather loosely in their seat pockets so that they have enough play to compensate for manufacturing tolerances and other irregularities and still seal effectively against the gate assembly. However, if the seats are able to float in an unrestricted manner in their pockets, they can move inwardly far enough to bind against the gate and segment when the gate mechanism is collapsed. This results in excessive operating torque of the valve and increased wear on the various components, particularly the sealing surfaces of the gate assembly and valve seats.
Various types of arrangements have been proposed to counteract this problem by limiting the extent to which the seat can move inwardly toward the gate assembly. For example, U.S. Pat. No. 3,823,911 to Natho et al provides seat spacers which maintain the seats far enough apart that they do not contact the collapsed gate assembly. U.S. Pat. No. 4,116,419 to Diehl et al utilizes seat retainers which serve as stops to maintain the valve seats outwardly of the collapsed gate assembly. Although these arrangements and others function in a satisfactory manner for the most part, they are not wholly without problems. If the means for controlling the floating movement of the seats is unduly complex, it adds significantly to the cost of the valve and the assembly difficulties. Increasing the size of the valve body is also undesirable in that the cost increases with increasing body size. In addition, removal of the seats must not be unduly difficult or time consuming since it is not uncommon for the seats to require repair or replacement.